Many types of air filtering systems and devices have been used previously for removing airborne particulate matter. Some of such systems are large and bulky and are not generally intended for use in offices or other areas where they may have to be moved occasionally.
Although so-called "table-top" air cleaners have been previously available, they generally are not very effective in trapping air borne particulate matter. Their collection rates typically drop rapidly over time. Air cleaners which use metal plates as collectors lose efficiency unless the plates are kept very clean. Also, such prior devices have the potential to generate ozone, which is very undesirable. They also are noisy in operation if they are capable of moving very much air through them. On the other hand, if they do not move very much air through them, then they cannot be very effective in removing particulate matter from the room in which they are used.
Some prior air cleaners utilize sharp metal needles in their ionizer system. The needles require periodic cleaning. They also decrease in efficiency with the passage of time. Further, the needles are sharp and present a safety hazard.
The air cleaner described in U.S. Pat. No. 4,376,642 (Verity) does not provide a self-contained filter module. Rather, a filter sheet is laid over a corrugated filter carrier secured to the frame of the air cleaner. In order to put in a fresh filter sheet, the dirty filter sheet must be lifted off the carrier and discarded. This is a messy and cumbersome task, and the dirty filter sheet is unsupported when it is removed from the carrier. Installation of a fresh filter sheet is also difficult, and air will leak around the edges of the filter sheet if it isn't properly installed.
Further, the ionizer in the Verity device is a needle which is located externally of the air cleaner housing. Also, the air cleaner does not include sound suppression features to quiet the operation of the air cleaner.
Another air cleaner is described in U.S. Pat. No. 3,999,964 for use in combination with a forced air heating system. It is not a portable air cleaner, but rather it is intended to be installed in the return duct system upstream from the inlet to the furnace. The cleaner includes an electrostatic cell having an ionizing section and a collecting section. In the ionizing section, foreign particles in the air pass through electrostatic fields to obtain an electrostatic charge. Then the charged particles are withdrawn from the air stream by the collector section. A foraminous filter media is positioned between the ionizing section and the collector section. The filter media is not electrostatically charged nor is it grounded.
There has not heretofore been provided a portable air filter system which is highly efficient and quiet in operation.